1. Field of the Invention
The present invention relates to a tool and a method of making a tool for breaking or excavating material, such as asphalt or coal, comprising a tool body and a cutting insert secured thereto.
2. Background of the Related Art
Tools of the type in question are usually mounted rotatably in a tool holder, which in turn is attached to an excavating machine, such as a road planing machine or a mining machine. Due to its rotation the tool is self-sharpening. A machine of this type is disclosed in U.S. Pat. No. 4,302,055, the disclosure of which is hereby incorporated by reference.
Various styles of rotatable cutting tools with a diamond enhanced inserts are known for use in mining and construction operations. These tools are known as "Point Attack Tools" or "Conical Tools. " When this kind of cutting tool is being used, a common problem is the wear of the material around the diamond enhanced insert As this wear progresses, eventually the insert will lose its support and come loose from the tool body to which it is attached. This problem is called "steel wash" around the insert.
For breaking or excavating of hard wear resistant materials, for instance for milling in poured asphalt (mastic), tools are used of the type shown in FIG. 1. This tool comprises a tool body 10A of steel and a cutting insert 11A of hard metal. The. cutting insert 11A is provided with a conical tip portion 12A and a shoulder 13A which is intended to rest against a supporting surface 14A on the tool body 10A. A wide based cutting insert of this type is known as a "cap." The rear contact surface 20A of the shoulder 13A is brazed to the supporting surface 14A. The cutting insert 11A is provided with a conical intermediate portion 15A which is located between the tip portion 12A and the shoulder 13A. The portion 15A protects the portion of the tool body 10A surrounding the cutting insert 11A from such wear that would cause the cutting insert 11A to get loose. When wear resistant material is excavated, for instance during milling in poured asphalt, the tip portion 12A becomes bluntended upon some wear of the cutting insert 11A. This wear increases the required cutting force.
One way of decreasing the cutting force required for worn tools would be to use a tool of the type shown in FIG. 2. Since the cutting insert 11B has a smaller diameter than the cutting insert 11A the cutting force is reduced. However, this should mean that the portion 16B of the tool body 10B surrounding the cutting insert 11B would be rapidly abraded, thereby causing the cutting insert 11B to loosen. Thus, cutting inserts of the type shown in FIG. 2 are suitable for use solely where the hard metal determines the life of the tool, for instance milling in concrete.
Another way of protecting the steel is to place a ring of carbide around a carbide insert shaped like a bullet such as the insert 11B. This combination basically fuictions the same as the "cap." A problem with both the "cap" and the combination of the insert and the ring is that the very top of the insert wears down. When this happens the whole top geometry becomes "blunt" and the cutting forces and the dust generation increase.
In order to overcome this problem, it has been suggested to keep the tip of the tool sharp by enhancing the carbide material of the tip with a layer of diamond material. A diamond coating layer provides improved cutting efficiency, reduced dust generation, less risk for frictional ignitions, etc. However, a common problem specifically related to these diamond coated tools is the fact that the diamond structure cannot withstand temperatures higher than about 650.degree. C. which might be present during the process of brazing the cutting insert to the tool body. This problem can be solved by use of a "heat sink" during the brazing process to keep the area containing diamonds at a lower temperature.
A second problem with diamond coated cutting inserts is that if the diamond enhanced insert is top brazed to a hardened steel body, the steel will get a tempered zone with lower hardness just below the insert. In this tempered zone with lower hardness the wear rate will be higher and the risk of losing the insert is increased.
The cutting insert is formed with a generally conical tip portion and provided with a shoulder which is intended to rest against a supporting surface on the tool body. The cutting insert according to the present invention provides a wear-resistant tool of the above type which requires a low cutting force at the same time as it is ensured that the risk is low that the cutting insert will get loose even during working in wear resistant material.
The invention is described in detail in the following with reference to the accompanying drawings, in which three embodiments are shown by way of example. It is to be understood that these embodiments are only illustrative of the invention and that various modifications thereof may be made within the scope of the claims.